The present invention relates to a semiconductor device, and more particularly, to a zero capacitor random access memory (RAM) which can prevent the reliability thereof from being deteriorated due to the application of a high voltage to drains in addition to a method for manufacturing the same.
As the semiconductor device design has progressed toward high integration, high speed operation, and low power consumption, the focus of the research then has been to achieve stable operational performance in a highly integrated device.
For example, one area of research is directed to a zero capacitor RAM (hereinafter, referred to as a “Z-RAM”) in which a plurality of carriers are charged up in a capacitorless floating body so that it is possible to write and read data through changing the threshold voltage (Vt) of a cell.
Since the Z-RAM has no capacitor, the capacitor forming processes are not required in a Z-RAM, and the capacitor forming areas are not needed in a Z-RAM. Therefore, when compared to a typical DRAM, the number of required processes advantageously decreases and the density of the Z-RAM increases.
The Z-RAM is realized using a silicon on insulator (SOI) substrate composed of a silicon substrate, an embedded insulation film, and a silicon layer, rather than a single crystalline silicon substrate made of bulk silicon. The Z-RAM operates as a high potential positive voltage. That is, when a high voltage is applied to drains and hot carriers are produced, pairs of electrons and holes are generated due to impact ionization by the hot carriers. Among the pairs of electrons and holes generated in this way, the electrons exit to the drains by the high voltage applied to the drains, and the remaining holes are stored in the silicon layer serving as the floating body. In a Z-RAM, data can be memorized by the fact that the threshold voltage (Vt) of a transistor varies depending upon the amount of holes stored in the floating body, namely, in the silicon layer.
However, a conventional Z-RAM is susceptible to abnormal operations due to presence of the high potential positive voltage (that is, because the high voltage must be applied to the drains of the ZRAM to generate the holes), thereby deteriorating the reliability of the Z-RAM.